(a) Field of the Invention
The present invention relates to a method for patterning an inorganic oxide film, and an electronic device including a mesoporous inorganic oxide film patterned by the method.
(b) Description of the Related Art
Inorganic oxides having large band gaps are used in photoelectrodes, sensors, anode active materials for secondary batteries, semiconductor materials and the like, and particularly, patterned inorganic oxide films bring about an increase in the efficiency of photoelectrodes and a nanowire field effect, so that patterned inorganic oxide films are useful in nanowire field effect transistors (FET), triisopropylsilylethynyl pentacene (TIPS-PEN) thin film transistors, and the like (Jr. H. He et al., J. Phys. Chem. B, 2006, 110, 50-53; and Meredith J. Hampton et al., Adv. Mater. 2008, 20, 2667-2673). Even in the application of these inorganic oxide materials in solar cells, patterning is very important.
For most of organic solar cells, in order to increase the efficiency, research has been actively carried out to change the structure or to develop electrolytes, dyes, inorganic oxides, organic molecules, polymers, electrode materials and the like that can exhibit excellent efficiency. However, most of solar cells cannot effectively utilize a large amount of solar radiation due to the limitations on the structure and materials that constitute the solar cells, and make use of only a certain amount of light to convert the photo energy to electric energy, while transmitting or reflecting the rest of light. Therefore, current solar cells have limitations on exhibiting high efficiency.
For this reason, intense research has been in progress to find technologies which allow more sunlight to be collected from a fixed area and thereby increases the efficiency of solar cells, namely, light harvesting technologies. For instance, technologies for harvesting light in dye-sensitized solar cells through methods such as photonic crystals (Guldin, S. et al. Nano Lett. 10, 2303-2309 (2010); Mihi, A. et al. Angew. Chem. Int. Ed. 50, 5712-5715 (2011)), a plasmonic effect (Ding, I.-K. et al. Adv. Energy Mater. 1, 52-57 (2011)), formation of various nanostructures (Munday, J. N. et al. Nano Lett. 11, 2195-2201 (2011); Yang, L. et al. Adv. Mater. 23, 4559-4562 (2011)), and use of improved electrolytes (Wang, M. et al. Adv. Mater. 20, 5526-5530 (2010)). However, dye-sensitized solar cells that are produced by the aforementioned methods are produced by complicated production methods that include costly processes, and have a disadvantage that those solar cells cannot be readily applied to the processes for mass production, and are not applicable to various systems.
In this regard, as a method by which the efficiency of a solar cell can be effectively increased in a simpler way, a method of forming a nanostructured pattern on an electrode has been suggested. This method is a method of collecting sunlight using the properties of light such as diffraction and reflection by means of a regular nanopattern arrangement formed on an electrode, and this method has been applied to organic molecule junction type solar cells (Na, S.-I. et al. Adv. Funct. Mater. 18, 3956-3963 (2008)) and inorganic silicon solar cells (Battaglia, C. et al. Nature Photon. 5, 535-538 (2011)).
However, since the photoelectrode of a solar cell using an inorganic oxide, such as a dye-sensitized solar cell, is usually produced using a composite paste which contains nano-sized inorganicoxide fine particles together with a solvent and a binder, it has been not so easy to form a nanopattern.
Methods of wet processes including photolithography and etching, which are well known as methods for patterning inorganic materials, are associated with complicated processes and enormous costs, and it is not feasible to produce micro-sized or nano-sized patterns and to use them in mass production by using a patterning method using arrays, nanowires, nanotubes or the like (Jr H. He et al. J. Phys. Chem. B, 2006, 110, 50-53; and Meredith J. Hampton et al. Adv. Mater. 2008, 20, 2667-2673). Furthermore, in a method of performing patterning using an elastomer mold of polyurethane acrylate (PUA) or polydimethylsiloxane (PDMS) with a solution containing a precursor by a transfer mold (LB-nTM) method, or direct patterning technologies such as a micromolding patterning method utilizing the capillary phenomenon and a gravure printing method, since a solution phase is utilized, the solution must be solidified before being transferred to a substrate in order to prevent liquid diffusion, and because a product is obtained in a bulk form, it is difficult to produce a porous structure with a large surface area and satisfactory characteristics. Also, there are limitations on the substrate that can be processed, and the working efficiency is low.
Particularly, in order to form a pattern with an inorganic oxide film of WO3, V2O5 or the like, or with an inorganic oxide that is utilized in photoelectrodes, such TiO2 or ZnO, pattern production should be carried out while a mesoporous state is maintained. However, in patterns produced by the methods described above, since the difference in surface energy between the mold and the solution is so large that micropattern production at a nano-sized level is not easy. As a result, there are limitations on the increasing of the efficiency of photoelectrodes.
Furthermore, in a method of forming a pattern by depositing a metal reflecting layer and subjecting the layer to nano-imprinting, it is difficult to effectively produce surface asperities on the top surface in a large size, and there is a disadvantage that mass productivity is poor, and the process becomes more complicated. With regard to a method of patterning a photoelectrode using lithography, since expensive materials are used in large quantities, and expensive processes such as a light interference UV process and strict control thereof are required, the method is inefficient in mass production. Also, a method of forming a ZnO nanopattern on top of a transparent conducting oxide (TCO) layer has a disadvantage that the structure is incapable of energy conversion with high efficiency because the thickness of the part in the solar cell structure which absorbs light and converts the light to electricity is too small.
Furthermore, a method of forming a nanopattern by lithography using a TiO2 precursor solution instead of nanoparticles has been suggested as a method for forming a pattern on a photoelectrode of a dye-sensitized solar cell. However, a photoelectrode that is formed by the method described above has a very small surface area and a small thickness, so that the photoelectrode exhibits an efficiency of 1% or less (H. Tokuhisa, P. T. Hammond, Adv. Funct. Mater. 2003, 13, 831-839.).
Recently, a technology of producing a nano-sized pattern by directly patterning a paste containing nanoparticles using a nanostamp formed of quartz has been suggested. However, it is difficult to produce quartz nanostamps, and since patterning is achieved under pressure, the nanostamps are susceptible to breakage and are not capable of producing regular pattern arrangements. Due to such problems, there has been suggested a method of coating the pattern with silver to utilize the plasmon phenomenon in connection with the method described above. However, this is not a technology that directly uses a nanostructure to harvest light, and since the technology involves coating of expensive silver through vacuum deposition, there are difficulties in putting the technology into commercial use (I.-K. Ding, et al., Adv. Energy Mater. 2011, 1, 52-57).
Therefore, there is a demand for the development of a photoelectrode which can utilize an effective light harvesting technology in order to increase the efficiency of solar cells, and there is a need to develop a technology capable of forming a nanopattern simply and effectively for the purpose of increasing the efficiency and enabling mass production. Also, there is an urgent demand for the development of a process for producing a photoelectrode for light harvesting, which can be applied to various dyes such as organic dyes as well as quantum dot dyes, and can be applied to solar cells which use various inorganic oxide particle pastes, electrolytes, electron and hole transfer materials and the like; and a process for patterning an inorganic oxide film for electronic devices.